Servo mechanism



Oct. 13,1953 sso 2,655,133

" SERVO MECHANISM Filed March 28, 1950 ZSnnentor O LENNART PAM. PALSSO/V Patented Oct. 13, 1953 OFFICE atta n. .SEIWQM HAn -Qs n;

errh aiio tmnhtn 1950.,

in .s tden v l-"ga 1949 1? Glaiur; (o1. 121. 41)

This invention relates to servomechanism and. more particularly to a novel hydraulically openated servomechanism wherein a controlling: ele-v ment when actuated brings about cor-respondingv actuation ofa controlledelementthrough changes in fluidipressure independently of the viscosity. of the fluidemployed;

Eluid operated: servomechanisms as heretofore constructed have operated-satisfactorily under certain conditions bLltbIlOt' under-others; such for example as whenachange in. temperature or. the like has: affected the viscosity of the operating; fluid.

It is thereforea .principalobjectof the invene tion to provide fluid actuated servomechanismso designed and constructed. as: to operateeatisfac: torily independently-of the viscosity org-the open-.- ating fluid; and thus capable of producing; ac: curate correspondence of. positioning of. the con: trolled element. to movements of the controllin element notwithstanding changes which mares..- cur in the viscosity of'said fiuid.

ether obiectapurposes and advanta es of the invention will hereafter more fully appear orwill, be understoodfzro the-following. description of; one embodim nt. of itdia mmatieally illu trated in the acc mpan ing. drawing which. shows. inits s l figure. partly in a transverse section and partly in elevation the. essential elementsthereof,

In t s id drawin which rep sents. perhaps 30 n of th o t eadily unde s andab e terms-of h ve ti there is llustrated a ma nhous n I pro ided itheerta n fluid. ehambersandwpasa es. as l erein fter be mo e hilly des ibed. nd it wil be nderst od ha mea s inst-sho n). t n ma l em l ed f9 upp yi g:ml hfi' mlse in thr h a c t 2 a subst ntiall constan how of operat n flu d wh h i exhaus ed o al otted to e a e eref-r m hro h a nda t3- Th passages n ch be in t eheu ine; h ou h whi he lu d fl -be e n th setw -norts n:- clude a r ct linea pass e 5. a fixed t rottle chamber 6 a nte media e assa e l. va iable t o e ha b 8, h n e o ca it raham: ber of a movable t n 0 and t exhaust. po ll O th Pi t n-W1? with exhaust port 3 of the. J I A p a e 4 nnecting-n chamb r 5 in th hen ne h ssa ef alleW -th -head ressureo the u d s l to b mai. ained against one si e of a piston 16 in this-chamber while-an inter;- mediate chamber I1 closed by a plug 118 and in.- terposed between. chambers 8 and I5 is connected with exhaust port 3 of thehousing through a port [-9 to maintain the exhaustzpressure of the-fluid- 5,5

2. in chamber H which also contains controlled element 20 0i? theservomechanism shown as em braced therein by pistons l0 and I5, this element being arranged to. actuate by suitable means hereinafter described the mechanical or other element to be-influenced by'its movements in response tothose of thecontrolling element 25; thus-for example-element 20 or its: equivalent may operate a valve to control admission of fluid pressure-toa cylinder for actuating a piston therein.

Reference has been made tofixed and movable throttles, and these regulate the flow of fluid through the mechanismto assist in causing the controlled'element to respond accurately to move-. ments of the controlling element and the structure and functions of these throttles will now be more fullyexplained. The fixed throttle in chamber 6 comprises-a collar 26 having a hole 21 at its center surrounded by a short, thin-walled tubelaiorthe like projecting into chamber 6 and terminating a predetermined distance from the end. face 29 of a rod 3i! which projectsfrom. the inner faceof a plug 3| forming a closure for the chamber, the edge of the tube and the rod face thus defining an annular fixed throttle port through whichifiuid can flow from chamber 6 into tube-28 at a rate proportional to the differential pressures and thence through passage 1 to the variable throttle chamber 8. The controlling element. 25 projects into the latter and. is provided with a planefendface 32 lying adjacent the end of a thin-walled'tube 33. preferably as closely similar as practical to tube- 28: and providing from the annular variable throttle port it defines with element 25a passage 34 into the interior cavity 9 f-piston t0; the flow of'fiuid through this port is of course correspondingly a function of the differential pressuresv and the spacing of element zemmhe adjacent end of tube 33 which changes momentarily as theformer is moved.

flesirably the effective area of piston I0 is about twice asg-reat as that of piston l6 although this precise relation is not essential provided piston Ill is always the larger in area, and when'the mechanism. is. in. operation with fluid being supplied under predetermined constant pressure through port 2', piston It} is normally subjected to a smaller pressure differential per unit area tending to move it toward piston [6 than is the latter in. being urged toward piston l0.

Thus if the effective area of the larger piston I10; is B" surface units and that of smaller piston l6:- isX Ftsurface units where X has any positive value lessthan unity, when full fluid pressure? is applied tothe mechanism through pain "a force of X F P force units is exerted through passage i4 and against piston is in chamber 15. This force is opposed by fluid pressure in chamber 8 tending to move piston 55 and controlled element 26 in opposition to piston It so that when these parts are in equilibrium the effect of the throttles on the flow of fluid through the mechanism is such that the force exerted against piston 10 opposing that exerted in chamber 15 against piston It may be expressed as FXP' force units. Disregarding the relatively negligible effects on the pistons of the pressure in chamber H, to maintain equilibrium F P' must equal X F P and consequently P=X P which demonstrates that the ratio of the effective areas of pistons it and i6 is an index of the ratio of the pressures on opposite sides of the fixed throttle during equilibrium conditions in the system and that P is less than P.

The line pressure P exerted back of piston it is as noted desirably relatively constant; consequently when for any reason the pressure in chamber 8 falls below the lower value P, piston I 6 can move under the influence of pressure P to correspondingly move controlled element 20 and piston iii in a direction to restrict flow of fluid through the variable throttle from chamber 3 into piston it. When the pistons are so proportioned that their effective areas have the relation mentioned, as when the effective area of piston H3 is twice as great as that of piston it, the said movement of both pistons and the controlled element as a unit continues until the rate of fluid flow through the variable throttle has been sufficiently restricted to build up pressure in chamber 8 equivalent to the arithmetic mean of the line pressure in passage and the exhaust pressure at port 3, re-establishing a condition of equilibrium wherein the flow of the fluid through both throttles is equalized, regardless of its viscosity, and P again equals XXP.

Conversely when the pressure in chamber 8 becomes greater than said lower value P, since the effective area of piston it is larger than that of piston I5 the former can move the latter and the controlled element 20 in the opposite direction until through the accompanying widening of the annular port of the variable throttle the excess pressure is relieved, the pistons and controlled element stopping when the pressure in the chamber reaches the value of XXP as equilibrium conditions are again restored.

It is thus evident motion of controlled element 26 results from change in pressure conditions in chamber 8 relatively to those in passage 5 and at port 3 and that in the mechanism here being described equilibrium conditions obtain when the fluid pressure in the former corresponds to the arithmetic mean of the pressures in the latter whatever be the viscosity of the fluid employed. Consequently, assuming equilibrium conditions to obtain with the controlling and controlled elements stationary, if controlling element 25 is now moved to a new position thereby either to enlarge or restrict the variable throttle port the pressure in chamber 8 is immediately correspondingly either decreased or increased as more or less fluid than is admitted to it from the fixed throttle can flow through the variable one; thereupon the unit consisting of pistons l0 and l 6 and controlled element 28 under the influence of the pressure differential on the pistons immediately moves correspondingly to the movement of controlling element 25 and promptly restores equilibrium conditions with the controlled element in a new position spaced from its former one the same distance and direction as the new position of the controlling element is spaced from its former one.

It is especially to be noted that substantial independence of the viscosity of the fluid is attained in the operation of the device, in part through the construction of the flxed and variable throttles, each of which is formed by a relatively thin walled tube of appreciable length, or a part of similar shape, cooperative with a plane surface and defining therewith an annular port having on the side adjacent the tube minimum radial width whereby under equilibrium conditions the same volume of fluid flows through both throttles per unit of time despite differences in absolute pressure conditions, and when the equilibrium is disturbed by motion of the controlling element it is automatically restored, with corresponding motion of the controlled element, regardless of the viscosity of the fluid supplied, which therefore may change from time to time without noticeably affecting the operation of the mechanism.

As above mentioned the controlled element can be adapted to perform any desired appropriate operation, as to open or close a valve or transmit motion to any other mechanical element, similarly to the controlled elements of servomechanisms generally as heretofore known.

Apparatus controlled by the servomechanism may for example include an ordnance gun provided with auxiliary equipment for moving the gun barrel in correspondance to motion of a sighting element (not shown) to which the controlling element 25 may be connected for movement with it. The element 25 through controlled element 20 thus directs a servo piston 35 movable in a cylinder 35 and having at its outer end a clevis 3'! for connection with the gun to supply the force for moving the latter. The cylinder 36 is provided with an inlet pipe 40 connected with a source of fluid under pressure, conveniently the same source as that which supplies port 2, and piston 35 therein is proportioned to expose to pressure in the cylinder on opposite sides of sealing rings 4! differential surface areas preferably bearing an area relationship of 2:1. The cylinder is connected thru pipes 2, 43 with fluid passages 66, 15 in the main housing l and further pipes id, 3'! connect these passages with ports 48, 49 respectively in a bafile 50 which the directed element 28 slidably engages. A by-pass recess 51 in the latter in one position of the element connects ports 48, 49 together and in another position connects the latter of these ports with outlet port 52 connected with a pipe 53 adapted to discharge fluid into chamber l1 and thence through port l9. The drawing shows element 2!! in a neutral position with port 49 disconnected from ports 48 and 52 and when the element moves up from that position in response to upward movement of element 25 port 49 is connected through by-pass 5: with outlet port 52 thereby allowing the pressure applied through inlet 45] to move piston 35 to the left since element 26 then obstructs port 48 and prevents escape of fluid from the right hand end of cylinder 36. Moreover when element 20 moves down from the position illustrated ports 48, 49 are connected together through by-pass 5| and pressure in cylinder 36 is immediately equalized on opposite sides of piston 35 at about the pressure of the source supplying fluid through inlet pipe 40. As noted the areas of the piston against which this pressure reacts are in a 2:1 ratio favoring movement of the piston toward the right when the pressure in the left hand end of the cylinder is more than half that in the other end. so the piston quickly responds by moving toward the right when the directing element 25 and hence controlled element 29 moves downward.

While I have herein diagrammatically shown and particularly described one embodiment of the invention it will further be understood I do not desire or intend thereby to be limited or confined in any way, as in the adaptation of the said or other embodiments thereof to particular uses and conditions changes and modifications in the form, structure and relationship of the several parts and instrumentalities employed will readily occur to those skilled in the art and may be made if desired without departing from the spirit and scope of the invention as defined in the appended claim.

Having thus described my invention, I claim and desire to protect by Letters Patent of the United States:

An hydraulic servomechanism to control an arbitrary body, which mechanism comprises two antagonistic pistons of different areas, and with opposite actions, the smaller being actuated by a particular working pressure, and the larger being actuated by a pressure resulting between. two throttles, connected in series, through which pressure fluid from said working pressure is adapted to flow on its way to an arbitrarily chosen lower outlet pressure, one of said throttles being fixed, and the other being responsive to impulses from both controlling and controlled elements, characterized by the members constituting each of said throttles having the same mechanical design and adapted to maintain identical flow conditions at both throttles when their positions are similar irrespective of the viscosity of the fluid, and by the area of the larger piston being a multiple of that of the smaller one.

LENNART PAUL PALSSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,585,529 Bovine May 18, 1926 1,822,667 Proell Sept. 8, 1931 2,110,622 Fischel Mar. 8, 1938 FOREIGN PATENTS Number Country Date 507,460 Great Britain June 15, 1939 539,068 Great Britain Aug. 2'7, 1941 

